Glutamate as a CNS transmitter. I. Evaluation of glucose and glutamine as precursors for the synthesis of preferentially released glutamate.

Slices of the molecular layer of the dentate gyrus of the hippocampal formation were incubated with either [14C]glucose, [14C]pyruvate or 14C glutamine and the efflux of endogenous and radioactive glutamate was monitored under various conditions. After prelabeling with either [14C]glutamine or [14C]glucose elevation of K+ concentration to 56 mM (Ca2+ free) increased efflux of endogenous and [14C]glutamate. Introduction of Ca2+ into the elevated K+ medium further increased the efflux of endogenous glutamate and radioactive glutamate derived from any of the precursors tested. In glutamine containing media, the increase in glutamate efflux as well as basal efflux was considerably higher than in the absence of glutamine and the specific activity of glutamate release was higher than that in tissue. Thus glutamine was superior to glucose or pyruvate as precursor and most specifically labeled the putative transmitter pool of glutamate. Similar experiments were carried out 4 and 14 days after a unilateral lesion in the entorhinal cortex which provides about 60% of the total synaptic input to the dentate granule cells. The Ca2+ dependent release of glutamate derived from either glucose or glutamine was markedly reduced on the operated side. This result suggests that the transmitter pool of glutamate is in perforant path terminals and can be synthesized from glucose or glutamine.

Glutamate as a CNS transmitter. II. Regulation of synthesis in the releasable pool.

Slices of the molecular layer of the dentate gyrus of rat hippocampus were used to study the regulation of glutamate synthesis and release. Net glutamate synthesis increased dramatically during conditions which stimulated the release of glutamate. The rate of glutamine incorporation into glutamate released into the medium was increased almost immediately upon stimulation with a 56 mM KCl, 3 mM CaCl2 medium. Synthesis appeared to be regulated both by glutamine uptake and the activity og glutaminase. Glutamine uptake was stimulated in the presence of 56 mM KCl and 3 mM CaCl2. The increased glutamine uptake was not due to a decrease in efflux, was unrelated to tissue glutamate levels, and could be dissociated from the rate of glutamate biosynthesis. The presence of Ca2+ ions and depolarization seemed necessary. Glutaminase activity was regulated by end product inhibition: increased levels of tissue glutamate resulted in a decrease in glutamate synthesis. Glutamine in the presence of 56 mM KCl increased the rate of glucose incorporation into glutamate over that seen without glutamine.

Effects of 5-fluorouridine on modified nucleosides in mouse liver transfer RNA.

Administration of the pyrimidine antimetabolite, 5-fluorouridine, to mice was found to cause a marked specific reduction of the amounts of 5-methyluridine, pseudouridine, and dihydrouridine but not of 3-(3-amino-3-carboxypropyl)uridine in tRNA from the livers of the treated animals. The data presented indicate that this effect is not simply due to the incorporation of 5-fluorouridine into tRNA; the drug appears to interfere directly with the enzymic reactions involved in the modification of the 5-position of uridine. 5-Fluorouridine was found to have no effect on the modification of adenosine, guanosine, and cytidine in mouse liver tRNA.

Effects of DL-ethionine on mouse liver tRNA base composition.

Treatment of mice with DL-ethionine and adenine causes a reduction of all methylated bases of liver tRNA. This effect is dose-dependent and specific for the methylated bases. Individual methylated components are affected to different extents, m22G being most sensitive to inhibition.